HISTOLOGICAL DEMONSTRATIONS. 95 



in the cerebro-spinal nerves, and not in the centres indi- 

 cate the division of the nerve fibre lengthwise into a number 

 of segments, each segment being apparently a transformed 

 cell. The gray sheath represents the cell membrane, in 

 which a single nucleus is to be found in each segment,, 

 while the axial cylinder and white substance may be 

 regarded as modified cell contents. 



149. The nuclei of the gray sheath are best brought into 

 view by silver staining. Place the sciatic nerve of a frog, 

 or the intercostal nerves of a rabbit, in \ per cent solution 

 of silver nitrate for two or three minutes, then treat in the 

 ordinary way ( 328). The nuclei are coloured brown; 

 another very characteristic appearance, however, is pro- 

 duced, viz. that of small brown crosses in the fibres. Each 

 cross results from the silver penetrating the axial cylinder 

 and staining it, together with the cement that covers it, at 

 the node where the medullary sheath is wanting. In such 

 a preparation, also, the simple layer of squamous epithelium 

 that covers the trunk of every cerebro-spinal nerve may be 

 seen, the epithelial outlines being silvered. 



150. Remove a portion of a lumbar or sciatic nerve of 

 a frog just killed. Fray out one end of the' nerve on a 

 slide with the point of a needle in a drop of salt solution 

 or aqueous humour. Cover the frayed portion only, and 

 examine. (H.) There is a double contour, that is to say, 

 a pair of lines on either side of the centre. The axial 

 cylinder is placed between the two pairs of lines, while the 

 interval between the outer and inner line of each pair is 

 occupied by the white and the gray sheaths. The gray 

 sheath may sometimes be found uniting the two ruptured 

 portions of a fibre, or stretching as an infundibuliform 

 process from a torn extremity. A double contour is not 

 perceptible in a living medullated fibre. Its appearance is 

 probably due to coagulation. 



If the fresh nerve be frayed out in water, the white 

 substance coagulates, and often assumes a fibrillated 

 appearance. 



151. Make a thin L. S. with scissors of the white 

 matter of fresh spinal cord of sheep or other animal. An 



